Preparation of cellulose acetate and the like



Patented Nov. 7, 1933 UNITED STATES:

PATENT OFFICE PREPARATION OF CELLULOSE ACETATE A AND THE LIKE James W. McKinney, Berlin, N. 1 L, assignor to Brown Company, Berlin, N. H., a corporation of Maine No Drawing. Application May 22, 1931 Serial No. 539,421

- 15 Claims. (01. 260-101) treatment is, therefore, an advantageous treat ment to apply to cellulose fiber, particularly to chemical wood pulp which is to constitute raw material for conversioni'nto cellulose acetate-for even when chemical wood pulp has been refined to high alpha cellulose content and-to pure whiteness, it invariably possessesa residual pentosan content higher than that of .good grade cotton.

When cellulose fiber is treated with alkali, however,1it-take's on a passive'state, especially as regards undergoing acetylationp'and this" is true irrespective of the derivation of the fibenthat is,

whether it is wood pulp or cotton; Some proposals have heretofore'been made regarding the restoration- 0f the fiber to an active condition, most of these proposals devolving about-the use of an acid as an activating agent.

After carrying out extensive and careful in vestigations concerning the'valueof various types of acids for activating cellulose fiber designed to undergo acetylation, I have found that lactic acid, is highly advantageous for this purpose, in

that not only does it activate the fiber, but it does so Without chemically degradingthe fiber ashigh viscosity for spinning into silk. and for deposition and setting as highly satisfactory films. This cannot be said of SllChaCldS as, the mineral acids, e. g., dilute sulphuricj acid, whose efiect is invariably bad, doubtless because ofv the degrading effect of V hydrolytic action on the cellulose.

While not limited thereto, my" invention has its greatest utility, so far as Iam aware, in making available chemical wood pulps for the prepa ration of cellulose acetates of thefinest quality. Heretofore, the raw material used for this purpose was almost exclusively cotton linters or papers made from high grade rags, such as shirt cuttings. Itis, of course, most desirable tobe able to utilize Further, the acti-,.

refined wood pulps both on account of their low cost and on account of the control which the pulpmaker (especially if he is producing a refined wood pulp) hasover such characteristics as viscosity of his product. On the other hand, cotton as found in nature, or rags, are apt to vary a good deal, which means that these materials must be carefully selected to fulfill the requirements of the cellulose acetate converter, for instance, the requirement of solution viscosity, which must fall within narrow limits. By alkali-treating cellulose fiber and then acid-treating it inaccordance with my invention, it becomes possible to utilize refined wood pulpsin making cellulose acetates comparable in their clarity of solution and other v qualities to cellulose acetates. made from choice cotton linters. Unless my invention is applied, chemical Wood pulps, even when highly refined .and reasonably expected to be as good as cotton linters, give cellulose acetates whose solutions are l or" high turbidity. My invention, when applied to bers other than refined wood pulp such as cotton linters or rag papers, makes possible much faster acetylation and greater uniformity in the character of the resulting cellulose acetate When practised in connection with prepared pulps of lower purity, such as sulphite pulp, the alkaline treatment, especially mercerizaticn, I effects a marked refinement of suchpulpsalthough,as of course would be predicted, such pulps, even when mercerized, are hardly as suited for making cellulose acetate as are woodpulps which are refined to begin with. g

a The alkaline treatment may be carried out with asolution of 'caustic soda or equivalent alstrength of alkaline solution maybe regulated according to the properties, especially viscosity, desired in the finished product, for I'have found that when the treated cellulose is converted into cellulose acetate by a standard method and the finished product is dissolved in acetone, the viscosity of this solution is controlled by the strength of the alkaline solution'which was used. I have found the best temperature for the alkalinetreatment to be 18 C., and the preferred range of strengths of alkaline solution to be pulp thickener or a centrifuge.

ment may be performed on the air-dry. or bonefrom about 8% to 12% NaOH, but these factors may be varied. When a caustic soda solution of the usual mercerizing strength (18%) is employed, an acetate or comparatively low viscosity is obtained. if a solution of, say, 10% caustic soda is used the'viscosity of the resulting acetate is somewhat higher; and at still lower concentrations of alkali, considerably higher viscosities may be obtained. The fiber may be used in bulk form in which case it is suspended in the al kaline liquor. Papers or sheets of fiber may be used, the latter form being that in which wood pulp is customarily stored and marketed. Thus,

so-called drier sheets of unbeaten, refined wood pulp of pure whiteness serve as'the starting material. rier sheets are comparatively thick, possessing a thickness say, 0.040 to 0.070 inches, and are fabricated by the wood pulp manufacturer on machinery of the papermaking type. They are the current form in which bleached sulphite pulp is sold to viscose-rayon manufacturers, as well as to paper manufacturers. The initial alkaline treatment of such sheets may hence be performed in apparatus of the same type as that used in the step of forming so-called alkalicellulose inviscose manufacture. Instead of keeping alkali associated with the sheets, however, in my process the alkali is removed therefrom by washing. The sheets are then pressed free of extractable water. fiber is handled similarly, afterthe alkaline treatment of my process. That is to say, the bulk fiber is Washed free of alkali, is then dewatered as much as possible, as by means of a The acid treatdry pulp, but preferably the moist pulp from the centrifuge or thickener is used directly.

Lactic acid .has been established as giving emi-,

nently satisfactory results, is non-volatile, and is soluble in water and such other common volatile solvents as alcohol and acetone. Suificient lactic acid is added as a solution in water'or some other suitable solvent, such as alcohol or acetone, to wet the fiber thoroughly; and then the water or other solvent is removed by ,evapo ration, leaving the concentrated lactic acid on the fiber. During the acid treatment, it is possible to heat the lactic-acid-treated fiber to hasten activation of the fiber for acetylation U without expelling the lactic acid, and thereby to benefit by the effect of substantially all the lactic acid initially associated with the fiber. It is again to be observed that alcohol or acetone may be used as the solventmedium for the lactic acid, or as the washing medium for the lacticacid-treated fiber, which medium can be readily removed from the fiber by volatilization before acetylation. Concentrated .(85%) lactic acid may be employed with good results, but one of the advantages of my process is that the dilute acid may be used and concentrated on the fiber by evaporation, thus reducing very materially the amounts of acid necessary to wet the fiber uniformly. Activation of the fiber is accomplished by the strong acid thus formed; and

while heating hastens the process, it is not necessary. Thus, if the water is removed by vacuum drying atroom temperature, activation is complete. The residual lactic acid does not interfere with subsequent acetylation and may therefore be left on the fiber; or, if there is any objection to storing or shipping .the fiber in the presence of this acid, it may be washed out with water or other solventmedia.

A bulk A specific example of procedure falling within the purview of the present invention may be practised about as follows. A pure-white, refined wood pulp of an alpha cellulose content of about 94% or greater may be used as a raw material. Assuming that the pulp is in the form of drier sheets, the sheets are subinersed at room temperature in a caustic soda solution of the desired strength until they are thoroughly saturated with the solution and uniformly reacted upon. The solution-soaked sheets of fiber may then be introduced into a water bath, wherein they may be disintegrated under vigorous agi- ,tation'to form a pulp suspension of, say, about 10% consistency. The pulp suspension may then be washed free from alkali, dewatered on a screen or in a centrifuge and pressed to remove residual free water. The pulp is then shredded and uniformly wetted with a dilute solution of lactic acid. To this end, the shreds may be immersed in a bath of a"5% solution of lactic acid (85% strength) in alcohol, and the excess alcohol pressed out. The acid-treated pulp may then be dried over a considerable period of time, say, for 16 hours at 100? C. The pulp may then be acetylated, either directly, or after residual lactic acid has been extracted or washed therefrom with a suitable washing medium, such as water or acetone, and 'ithas then been redried. Acetylation may be effected in the usual manner; andv the conventional after-- acetylation treatments for modifying the cellulose acetate into the acetone-soluble variety may be performed. Acetylation' takes place smoothly and rapidly. The finished cellulose acetate dissolves completely in acetone to give substantially colorless lacquers of excellent. clarity.

WhatIclaimis:

1. A product consisting of lactic-acid-treated wood pulp of an alpha cellulose content at least about 94%. I

2. A process which comprises treating cellulose fiber with a solution of alkali, then treating-the fiber with lactic acid, and acetylating.

3. A process which comprisestreating cellulose fiber with a solution of alkali, then treating the fiber with asolution of lactic acid,,removing the solvent vehicle from the .fiber, and acetylating.

4. A process which comprises treating cellulose fiber with a solution of alkali, then treating the fiber with a volatile solvent solution of lactic acid, heating the fiber to hasten its activation by the acid and to remove the volatile solvent, and acetylating.

5. A process which comprises treating cellulose fiber with a solution of alkali, then treating the fiber with a solution of lactic acid, heating the fiber to hasten its activation by the acid and to remove the solvent, extracting the lactic acid from the fiber, and acetylating.

6. A process which comprises treating cellulose fiber with a solution of alkali, then treating the fiber with a volatile solvent solution of lactic acid, heating the fiber to hasten its activation by the acid andto remove the volatile solvent, extracting the lactic acid from the fiber with acetone, and acetylating.

'7. A process which comprises treating cellulose fiber with a. solution of alkali, then treating the fiber. with a solution of about 5% lactic acid in alcohol, drying the fiber,v and acetylating.

8. A process which comprises treating refined wood pulp with a caustic soda solution of about 8% to 12% strength, washing the pulp free from alkali, treating the pulp with a lactic acid solution, and acetylating the pulp.

9. A process which comprises treating refined wood pulp with a caustic soda solution of about 8% to 12% strength, washing the treated pulp free from alkali while suspending it in water, dewatering the pulp suspension, shredding the dewatered pulp, covering the pulp shreds with lactic acid solution, removing solution in excess of that necessary to wet the shreds, and acetylating the lactic-acid-treated shreds.

10. A process which comprises treating refined wood pulp with a solution of alkali, washing the pulp free from alkali, treating the pulp with a lactic acid solution, and acetylating.

11. A process which comprises treating refined wood pulp with a solution of alkali, washing the pulp free from alkali, treating the pulp with a lactic acid solution, washing the pulp free from lactic acid, dryin and acetylating.

12. A process which comprises refining chemical WOOd pulp in a solution of alkali, treating the refined pulp with lactic acid, and acetylating.

13. A process which comprises refining cellulose fiber in a mercerizing solution of alkali, treating the refined fiber with lactic acid, and acetylating.

14. A process which comprises refining chemical wood pulp in a solution of alkali to an alpha cellulose content of at least about 94%, treating the refined pulp with lactic acid, and acetylatmg.

15. A process which comprises refining chemical wood pulp in a mercerizing solution of alkali to an alpha cellulose content of at least about 94%, treating the refined pulp with lactic acid, and acetylating.

JAMES W. MCKINNEY. 

